In general, the horizontal synchronizing circuit is a circuit designed to synchronize the horizontal deflecting operation of a television receiver set with a horizontal synchronizing signal of a television signal and is, in principle, constituted by a phase-locked loop (PLL) including series-connected phase detector, low pass filter, voltage-controlled oscillator and horizontal deflecting circuit. The phase detector serves to detect the difference in phase between the horizontal synchronizing signal and a flyback pulse generated from the horizontal deflecting circuit and to generate a voltage signal corresponding to such phase difference. This voltage signal is applied to the voltage-controlled oscillator through the low pass filter to cause the voltage-controlled oscillator to control the oscillating frequency according to the voltage signal and then to generate a pulse synchronized with the horizontal synchronizing signal. The horizontal deflecting circuit is driven in timed relation to this pulse. As a result thereof, the fly back pulse synchronized with the horizontal synchronizing signal is generated from the horizontal deflecting circuit.
In the construction described above, since the voltage-controlled oscillator can accommodate even a slight change in voltage signal from the phase detector if the low pass filter has a small time constant the oscillating frequency can be finely controlled. In other words, the oscillating frequency of the voltage controlled oscillator can satisfactorily respond to change in phase of the horizontal flyback pulse which takes place with change in brightness of the picture screen. However, since the low pass filter having a small time constant tends to permit the passage of noises which have inadvertently been outputted from the phase detector, the voltage-controlled oscillator responding to these noises generates an output signal of erroneous frequency.
Conversely, if the time constant of the low pass filter is increased, the possibility of the erroneous operation resulting from said noises can be eliminated, but it will become impossible to finely control the oscillating frequency of the voltage-controlled oscillator.
In order to obviate the above described problem, an arrangement (double PLL) wherein two PLL are connected has been proposed, and an improved version of this double PLL is disclosed, for example, in Japanese Patent Publication No. 53-15335 (corresponding to U.S. Pat. No. 3,891,800). According to Japanese Patent Publication No. 53-15335, the use of two horizontal oscillators each having a variably controlled oscillating frequency has been proposed to control the frequency of an oscillator preceding thereto in accordance with the difference in phase between an oscillating output thereof and the horizontal synchronizing signal and also to control the frequency or phase of an oscillator following thereto in accordance with the difference in phase between the oscillating output so controlled and the horizontal flyback pulse.
In such a circuit, the erroneous operation resulting from the noises contained in the horizontal synchronizing signal can be eliminated by selecting the time constant of a circuit operable to control the preceding oscillator to be of a large value and, by selecting the time constant of a circuit operable to control the following oscillator, it is possible to cope satisfactorily with change in phase of the horizontal flyback pulse which varies with change in brightness of the picture screen.
However, in the conventional circuit described above, since the two oscillator oscillate at the same frequency as that of the horizontal synchronizing signal (hereinafter, referred to as horizontal frequency), the range in which the frequency thereof can vary cannot be set to a sufficiently narrow range relative to the horizontal frequency. Because of this, in the case where a quasi-synchronizing signal having a frequency diverting from the normal horizontal frequency due to spurious interference and others is generated, there is the possibility that the horizontal synchronizing system tends to be stabilized at the diverted frequency. In addition, in the conventional circuit described above, as a method for controlling the following oscillator, there is employed a method wherein the horizontal flyback pulse is integrated to give a sawtooth voltage signal which is in turn supplied to a delay element to make a delay corresponding substantially to half the horizontal scanning period, the delayed voltage signal being used as a reference voltage signal to the phase detector. According to this method, the phase comparator is complicated in construction and, in addition, the delay element is not suited for the production in an IC.
The present invention has been developed with these points taken in consideration and has for its first object to provide a horizontal synchronizing circuit with a variable frequency oscillator capable of oscillating at a frequency sufficiently higher than the horizontal frequency which is utilized to avoid any possible erroneous operation of the horizontal deflection circuit which would result from the quasi-synchronizing signal due to the spurious interference, etc.
A second object of the present invention is to provide a horizontal synchronizing circuit wherein the output from said variable frequency oscillator is frequency-divided to enable a pulse located intermediately of the horizontal scanning period to be generated without the delay element being used, so that the timing at which the horizontal deflection circuit is to be driven can easily be controlled.
A third object the present invention is to provide a horizontal synchronizing circuit wherein a first gating pulse of a pulse width including the horizontal synchronizing pulse period is synthesized by the utilization of the output from said variable frequency oscillator, a horizontal synchronizing pulse gated by this gating pulse being so supplied to the phase detector as a reference input that, even when a so-called video-in-sink wherein a video signal component of a television signal received is swept into the side of a synchronizing signal occurs, it will not be regarded as a horizontal synchronizing signal.
A fourth object of the present invention is to provide a horizontal synchronizing circuit wherein a second gating pulse generated only during the burst gating period is synthesized by the utilization of the output from the variable frequency oscillator, a horizontal synchronizing pulse gated by the pulse which is an inversion of this gating pulse being so supplied to the phase detector as a reference input that any possible erroneous operation resulting from noises carried on a back porch of the horizontal synchronizing signal can be eliminated.
A fifth object of the present invention is to provide a horizontal synchronizing circuit which is simple in structure and can easily be manufactured in an IC.